1. Field of the Invention
The present invention relates to an output circuit to be used for a semiconductor integrated circuit and, more particularly, it relates to an output circuit of a CMOS type semiconductor integrated circuit designed to reduce the influence of noises that can appear on related power source lines when a plurality of such output circuits of the integrated circuit are simultaneously switched on or off for output signals.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional output circuit of a semiconductor integrated circuit. The semiconductor integrated circuit has the output circuit for outputting signals to the external devices. The signal in the integrated circuit (not shown) is sent out from output terminal 13 by way of a prebuffer circuit 11 and an output buffer circuit 12. CMOS inverters are normally used for the prebuffer circuit 11 and the output buffer circuit 12 in a CMOS type semiconductor integrated circuit. The output buffer circuit 12 of an output circuit 18 of this type normally comprises a P-channel MOS transistor 14 and an N-channel MOS transistor 15 having small dimensions (including the channel width) and hence a rather small current driving capability so that a large current may not flow, if instantaneously, between related power source lines.
As illustrated in FIG. 2, a large number of output circuits 18 are normally disposed in parallel between a high potential power source line 16 and a low potential power source line 17 in a semiconductor integrated circuit. Resistive, capacitive and inductive loads are parasitically existent on both power source lines 16 and 17. Therefore, when more than one of the output circuits 18 are simultaneously turned on or off for output signals, the level of the electric current flowing into the power source can enormously fluctuate within a short period of time to generate noise on either power source line 16 or 17, which may propagate and eventually appear on the output terminals of output circuits which are not turned on or off. Such noise is often called "simultaneous switching noise". In order to reduce the possibility of appearance of such noise, the MOS transistors 14, 15 of an output buffer circuit 12 of the type under consideration are made rather small. If, on the other hand, these devices are made too small, they can by turn reduce the switching speed of output signals. This tendency of reducing the switching speed can become remarkable when output circuits have a heavy load. An output circuit comprising small transistors and hence having a reduced switching speed cannot meet the requirement of high speed operation in the field of advanced electronic devices, meaning that any conventional output circuit cannot effectively eliminate the influence of simultaneous switching noise without undesirably reducing the load driving capability of the output circuit.